Coating for sheet steel in transformer cores



' May 23, 1961 COATING FOR SHEET STEEL IN TRANSFORMER CORES N. E. STONEFiled Jan. 11, 1957 Z3 23 23 I 1 -v. L T L W W &1 R L 1 T L L 2? I i L 393 3 MN j ig Mvm kcrb United States Patent COATING FOR SHEET STEEL INTRANSFORMER CORES Norman E. Stone, Pittsburgh, Pa., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis.

Filed Jan. 11, 1957, Ser. No. 633,664

2 Claims. (Cl. 336-60) This invention relates in general to woundmagnetic cores and in particular to wound magnetic cores provided withan insulating and spacing coating of a predetermined form.

Electrical steel is sometimes supplied to the transformer manufacturerin either a semifinished state or a fully processed state in the form oflarge sheets or rolls. After the material is processed to a sizeconvenient for the core manufacturing process it is given a hightemperature anneal in the case of the semifinished state which resultsin crystal growth in the preferred direction, further purification ofthe steel and in the case of the fully processed state, a lowtemperature anneal which results in relief of any strains set up in thematerial during the manufacturing process. During the annealing processthe magnetic steel material is in the form of a coil or rolled strip.Thus, there is relatively large surface contact between the layers ofthe magnetic material while in the annealing furnace. Due to theplasticity of the steel at the elevated temperatures associated with therecrystalization anneal, there is a tendency for the associated layersin the assembly to stick together and possibly even weld to one another.It, therefore, becomes desirable to have a coating with a refractorysubstance between adjacent layers during the high temperature anneal toavoid this sticking, and which will also provide an insulating coating.

It often becomes necessary, during the process of manufacturing woundcores to unwind the annealed rolled strip and wind it around the windingleg of a preformed coil winding. To facilitate this step, it is usuallydesirable to have a slight degree of spacing between the layers of thewound core prior to annealing it.

In other methods of making wound cores, this spacing is formed byinserting shims or spacing elements between the layers prior toannealing. Paper, thread, and other carbon elements have been used asspacing elements.

These materials burn or char at the annealing temperatures and thecarbon thus formed contaminates the annealers. Consequently, theannealers must be frequently cleaned. In addition, these carbon spacersare limited to use in low temperature stress annealing. If used in hightemperature recrystalization annealing, the carbon migrates to the woundsteel material and defeats the purpose of the recrystalizationannealing.

Wire has also been used as a spacer but has not produced a uniformdegree of spacing and has frequently resulted in grooves in the rolledsteel strip. During high temperature annealing the wire frequently fusedwith the steel strip.

Increasing the thickness of the insulating coating on the steel strip toact as a spacer has also been attempted. However, when building up largequantities of the coating there is less probability of obtaining auniform degree of spacing between the laminations of the wound steelstrip. Because of the increased area of the coating providing thespacing it is extremely diflicult to obtain a uniform coating surface.Thus a spacer of a noncarbon substance which 2,985,855 Patented May 23,1961 will not adversely affect the steel strip and which will provide auniform degree of spacing will overcome the aforementioned objections.

According to this invention such a uniform degree of spacing between thelaminations of the wound steel strip is obtained by covering the stripwith a coating comprising a refractory substance and a bonding agent.The exposed surface of this insulating coating is formed into raisedareas of spacing elements or spacers.

According to the present invention the insulating coat ing with spacerscan be applied to the top and bottom surfaces of the magnetic sheetmaterial by applying the coating substance to the surfaces and formingthis substance into predetermined positioned high areas of spacingelements.

It is therefore an object of this invention to provide a magnetic sheetmaterial with an insulating coating having raised areas.

A further object of this invention is to provide a wound core having auniform predetermined space between laminations.

A still further object of this invention is to provide a wound core withremovable spacing elements.

Another object of this invention is to provide an improved method ofbuilding wound cores with predetermined spacers between the laminationsin the core.

Objects and advantages other than those mentioned above will be apparentfrom the following description: Fig. 1 is a schematic view illustratingthe process of coating a magnetic sheet material for wound cores;

Fig. 2 is an isometric view of a grooved roll;

Fig. 3 is a top view of magnetic sheet material with an insulatingcoating having spacers; and

Fig. 4 is an enlarged cross sectional view of Fig. 3 on line IV-IV.

:In accordance with the present invention, magnetic sheet material or asilicon steel strip 11 is covered preferably on both top and bottomsurfaces with an adherent coating solution 12 thereon. This coatingcontains a finely divided refractory substance, a bonding agent, andwater.

Any 'known refractory substance such as lime, dolomite, magnesium oxideand silica, and various combinations of the above may be used. Apreferred bonding agent is carboxymethyl hydroxyethyl cellulose sold bythe Hercules Powder Company under the name of CMHEC. However, thisinvention is not limited to carboxymethyl hydroxyethyl cellulose asother bonding agents such as methyl cellulose, and an alkyd resin orcellulose acetate in suitable organic solvents may be used.

A preferred coating solution 12 may be prepared .by adding pounds ofrefractory substance, a mixture of calcium oxide and magnesium oxide,325 mesh, and 16.7 pounds of carboxymethyl hydroxyethyl cellulose to asufficient amount of water to form a slurry. These ingredients are thenground in a wet ball mill for several hours until the refractoryparticles are thoroughly dispersed. It has been found that a suitablecoating is obtained when the specific gravity of the solution issubstantially 1.07 and its viscosity is 18.5 in a .2 Zahn cup.

The coating solution 12 is then ready to be applied to the top andbottom broad surfaces of the sheet material 11. The resultingunsymmetrical exposed surface of the coating is then formed intorelatively high and low areas forming an insulating coating 21 with twodifferent thicknesses. The raised areas 22 are of a uniform height witha predetermined cross section and serve as spacing elements.

The sheet material 11 with its adherent insulating coating 21 is thenplaced in an oven 16 where evaporation of the excess fluid in thecoating occurs. As a result of evaporation the coating is transformedinto a solid form. The surface of the coating adjacent the sheetmaterial remains adherent and the exposed surface of the coating orsurface opposite the adherent surface be comes: nonadherent. conditionsuitable for handling and cutting as it's insulating coating 21 withspacers 22 will not accidentally flake off.

There are several ways of applying a coating, with spacers to a sheetmaterial and the following. described method is a preferred one. If thesheet material is obtained in large rolls the coating is applied to itstop surface as it unrolls by such means as a tank 13 with a spigotcontaining coatingmaterial 12. The unrolled sheet material is thenpassed between two circumferentially grooved coating rolls 14 which formcontinuous thin insulating coatings 23 and spacing ridges 22 having. adistinctly greater thickness. The roll applying the underneath coatingruns immersed in a vat containing the coating material.

The spacing elements 22 may be varied by changing the width, depth andpatterns of the grooves in the rolls. A suitable pattern is evenlyspaced circumferential grooves 19' of rectangular cross sectiontransversed by another groove 20. Diagonal grooves, holes, spiralgrooves, and crosshatched grooves are other suitable patterns within thescope of this invention.

The two coating rolls 14 are set apart at a predetermined distance witha small gap between each roll and the sheet material 11. One roll isplaced. in near contact above the sheet material and the other roll isplaced in near contact underneath the sheet material. As the sheetmaterial 11' containing the coating 12 passes between the grooved rolls14 an insulating coating. 21 com prising alternately arranged low areas23 and high areas of spacers 22 is formed on. the surfaces of the sheetmaterial. The depth of this insulating coating is dependent on thepositioning of the rolls 14. Thus, to increase the depth of the coating21 the rolls 14 are moved farther apart from each other and the sheetmaterial 11.

After being coated the material is passed through an oven 16 whichcauses the water in the coating solution to evaporate without harmingthe coating. The resulting insulating and spacing coating 21' adheres tothe magnetic material 11 so that it may be freely handled in Winding andother forming operations which occur before the final anneal without anysubstantial amount of the refractory substance rubbing E.

The strip material is then formed into a wound core 17 and. placed inthe annealing furnace 18. It is essential' that the spacers 22- arecapable of supporting. the weight of the laminations because theselaminations frequently become pliable and weakened during high annealingtemperatures. During annealing a portion of the insulating coating 21 onthe surfaces of the wound core decomposes freeing aportion of coating22, the freed portion having performed its function as. a spacer.Another The sheet material 11 is now ina.

portion of the adherent coating 23 reacts with the surface of the steelto form a final and permanent insulating bond. The insulating coating 21on the surfaces of the wound core decomposes during annealing causingremoval of the inert spacers. Thus the core 17 in a single operation isprovided with a permanent insulating coating and an easily removablespacer which provides uniform spacing between its laminations andconsequently may be rapidly assembled into the electrical coils. Thus,the wound core 17 having been annealed, is stress relieved and becauseof the spacers 22 has uniform space between its laminations andconsequently may be rapidly assembled into the electrical coils.

While only one embodiment of the present invention has been described,it will be apparenttothose skilled in the art that other modificationsmay be employed without departing from the spirit of the invention orfrom the scope of the appended claims.

What is claimed is:

l. A core with laminations of a magnetic sheet material, said sheetmaterial having thereon a coating comprising a composition of arefractory substance and a bonding agent, a surface of said coatingadhering to said sheet material and having a nonadherent surfaceopposite said adherent surface, saidnonadherent surface having raisedspacing areas for maintaining a spacedrelationship between saidlaminations.

2'. A wound core with laminations of a magnetic sheet material, saidsheet material having a coating on a'plurality of surfaces of saidlaminations, said coating comprising a" composition of a refractorysubstance and a bonding agent of corboxymethyl hydroxyethyl cellulose, asurface of said coating'adhering to said sheet material and havinganonadherent surface removed from said adherent surface, saidnonadherent surface having raised spacing areas for supporting saidlaminations.

References Cited in the file of this patent UNITED STATES PATENTS1,454,323 MacInnes May 8, 1923 2,261,983 Ford Nov. 11, 1941 2,289,339Brennan July 14, 1942 2,389,497 Gat Nov. 20, 1945 2,390,863 Amidon et a1Dec. 11, 1945 2,394,047 Elsey et a1. Feb.. 5, 1946 2,410,220v LangworthyOct. 29, 1946 2,501,349 Nagel 1 Mar. 21, 1950 2,561,462 Compton et al.July 24, 1951. 2,584,564 Ellis Feb. 5, 1952 2,655,101 Newman Oct. 13,1953 2,670,026 Ungar Feb. 23,- 1954 2,699,195 Weller Jan. 11,v 19552,864,065 Horelick et a1. Dec. 9, 1958 FOREIGN PATENTS 587;,374 GreatBritain Apr. 23, 1947

